The hydrotreating of petroleum feedstock serves to upgrade the hydrocarbon fraction in the eventual distillate. The purpose of this is to remove impurities such as sulfur, nitrogen, oxygen and organo-metallic compounds which may be present. By hydrotreating prior to cracking or reforming, the quality of the feedstock is improved and downstream cracking or reforming catalysts are protected. That is, the downstream cracking or reforming catalysts are not altered or poisoned, and thus, they may be easily recycled. The most common material which poisons petroleum conversion catalysts is sulfur. Hydrodesulfurization is also beneficial for subsequent combustion of the treated hydrocarbon fraction as fuel in domestic heaters, industrial furnaces, etc., in reducing the discharge of sulfurous combustion products into the atmosphere.
Petroleum refining is one of the largest manufacturing industries in the United States, with petroleum products accounting for at least 10% of the Gross National Product. Six basic catalyst-consuming processes are employed in the conversion of petroleum. The four most important are cracking, hydroprocessing/hydrodesulfurization, catalytic reforming and alkylation. Of lesser importance are hydrocarbon polymerization and isomerization, petrochemical processes such as the manufacture of aldehydes by reaction of carbon monoxide with hydrogen, the manufacture of alcohols by reacting aldehydes with hydrogen, as well as other vapor-phase catalytic processes. Hydrodesulfurization (HDS) is a term that is applied to the hydroprocessing of any petroleum feedstock since desulfurization via catalytic hydrogenation or hydrotreating is involved. However, the term is most appropriate for the treatment of the heavy, semi-solid residue or bottom stream from crude oil distillation towers.
Among the catalysts employed in hydrotreating and hydrodesulfurization are cobalt-molybdenum, nickel-molybdenum and nickel-tungsten combinations on an alumina support. The most common catalyst is a cobalt-molybdenum trioxide.
At the present time, it has been estimated that approximately 55 million pounds of hydrotreating/hydrodesulfurization catalysts are in use in the United States. The annual replacement rate of these catalysts has been reported to be approximately 20-21 million pounds of which approximately 10-11 million pounds are cobalt-molybdenum, approximately 9 million pounds nickel-molybdenum and approximately one million pounds nickel-tungsten.
When cobalt and molybdenum were in short supply and commanded a high price, the metals were frequently recovered from the spent catalysts. Even during periods of good metal markets, the efficiency of the recovery of cobalt and nickel was low and most of the active companies concentrated their efforts on molybdenum recovery. However, since the downturn in the economy and the oversupply of metals in 1980-82, economic factors as well as substantial technical difficulties involved have greatly reduced the incentive for the recovery of metals from the spent catalysts. Surprisingly, little effort has been made in the prior art in seeking to recover the inert alumina support which accounts for about sixty percent of the weight of spent catalysts.
One method by which the active metals in the catalyst have been recovered in the prior art has been by total digestion, i.e., by dissolving the catalyst in hot concentrated sulfuric acid. Although this method has the obvious advantage of being able to recover 100% of the available metals, it also has disadvantages. The amount of acid necessary for total digestion is very costly from an economic standpoint, and the resulting solution contains far more dissolved aluminum than active metals, making separation of the valuable active metals from the relatively inexpensive aluminum a difficult problem.
The current and growing quantity of spent catalysts is also of environmental concern. Not only do the spent materials tend to be pyrophoric but they also contain leachable toxic heavy metals such as arsenic in addition to the nickel, cobalt or molybdenum. Moreover, present commercial processes tend to recover only molybdenum or cobalt and leave the inert alumina matrix and unrecovered metals for land disposal. The prior art is lacking in an economical method to recover and reuse all significant chemical and metal values including the alumina matrix.
The present invention overcomes the problems and disadvantages present in the prior art by providing a simple, efficient and economical method for effecting a substantially complete extraction and recovery of valuable metal values from spent hydrodesulfurization or hydrogenation catalysts.